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Check My Math - Compression Ratio |
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AMXrated 
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Joined: Jul/26/2015 Location: Indiana Status: Offline Points: 290 |
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 Topic: Check My Math - Compression RatioPosted: Sep/19/2018 at 8:20pm |
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Working on a '72 360 that has been bored .030" over. Have a set of Wiseco Pro Tru Flat tops (p/n PTS539A3) with a stated CR of 11.22:1.
These pistons have a compression height of 1.59 which is less than stock at 1.601, so this would mean my deck clearance goes from .012 nominal to .022. Right? Assuming that is correct and I punch my numbers into the CR calculator from Diamondracing.net I get the following: Bore 4.11 Stroke 3.44 Rod Length 5.875 Gasket Bore 4.13 Gasket Compressed Thickness .045 Block Deck
Height 9.208 Top Ring Down .28 Chamber
Volume 57.92 Dome/Dish
Volume -8.6 Piston to
Deck .022 For the given
dimensions, compression ratio = 12.49:1 If I use a set of Scat connecting rods that have a length of 5.885, I get back the .010" lost from the shorter pistons and should be back to nominal deck clearance, right? And, if I punch all that into the calculator, I get: Bore 4.11 Stroke 3.44 Rod Length 5.885 Gasket Bore 4.13 Compressed Gasket .045 Block Deck
Height 9.208 Top Ring Down .28 Chamber
Volume 57.92 Dome/Dish
Volume -8.6 Piston to Deck .012 For the given dimensions, compression ratio = 12.89:1 This is probably a question for Wiseco, but why is their rated CR (11.22:1) different than what the calculator is coming up with? Or, do I have a bad number (or two) somewhere? Also, it appears the extra .010" on the Scat rods do not have an extreme effect on CR. If the 12.89:1 number is accurate, will I be able to run 93 octane in it? Just getting back into the hobby and it's been a long time since I did any real engine work. Any thoughts/opinions are greatly appreciated. Thanks. |
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304-dude
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Joined: Sep/29/2008 Location: Central Illinoi Status: Offline Points: 9082 |
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Posted: Sep/19/2018 at 8:32pm |
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Geeze, no way you can safely run 93 octane with over 12.5:1 compression. Many said my target 11.5:1 was pushing it.
Though I plan on a slightly rich fuel AFR to compensate, on 93 octane for street use. Our Honda can do regular 87 with 10.5:1 compression. The Acura version of our engine runs at 11.25:1 compression on premium 93 octane. |
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71 Javelin SST body
390 69 crank, 70 block & heads NASCAR SB2 rods & pistons 78 Jeep TH400 w/ 2.76 Low 50/50 Ford-AMC Suspension 79 F150 rear & 8.8 axles Ford Racing 3.25 gears & 9" /w Detroit locker |
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WesternRed
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Posted: Sep/19/2018 at 8:43pm |
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Do you have the piston dome entered correctly, usually positive for a dish/valve reliefs, negative for a dome.
4.265” gasket bore rings a bell to me. Edited by WesternRed - Sep/19/2018 at 8:46pm |
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I've finally given up drinking for good...........now I only drink for evil.
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AMXrated
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Joined: Jul/26/2015 Location: Indiana Status: Offline Points: 290 |
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Posted: Sep/19/2018 at 9:22pm |
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Wiseco's spec sheet lists Top Type as "Dome Top". Net Head Volume as -8.6. So, I think that is a good number.
You're right--little small on the head gasket bore...it changed the CR for the stock rod length to 12:37:1 and for the Scat rod length to 12:76:1. Still not sure why Wiseco's CR number is so much less than calculated. Thanks, Jack
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PHAT69AMX
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Posted: Sep/19/2018 at 9:27pm |
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Posted Values, 8.6 cc Piston Dish, spreadsheet comes up with this....
Nope, this is wrong, has Stock 5.875 Rod Length not 5.885 Scat Rod Length and unsure about Piston Compression Height... ( edit, poster says 1.59x CH is correct )
 Edited by PHAT69AMX - Sep/25/2018 at 11:10am |
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AMXrated
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Joined: Jul/26/2015 Location: Indiana Status: Offline Points: 290 |
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Posted: Sep/19/2018 at 9:29pm |
I was afraid of that...I do live in the Midwest and E85 is readily available, although 105 octane seems like it may be too high. Is there a calculator online somewhere that shows recommended CR to fuel rating? Thanks.
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PHAT69AMX
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Posted: Sep/19/2018 at 9:47pm |
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To get the Wiseco Stated 11.2x:1 CR... ( revised ) Bore 4.11 = Stock +.030 Stroke 3.44 = Stock Dome/Dish Volume 8.6cc = DISH / Valve Reliefs Piston to Deck 0.011 Gasket Bore 4.265 Gasket Thick 0.045 Chamber Volume 51.50 = 57.92 Stock Milled .015 Rod Length 5.872 = Stock -.003 Block Deck Ht 9.193 = Stock -.015 Piston Comp Ht 1.590 = Stock Wiesco A cam with an Intake Valve Closing Angle that bleeds off Static CR.
Edited by PHAT69AMX - Sep/19/2018 at 10:23pm |
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AMXrated
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Joined: Jul/26/2015 Location: Indiana Status: Offline Points: 290 |
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Posted: Sep/19/2018 at 9:50pm |
Your calculated compression height matches Wiseco's spec (1.59). I found a page from Wiseco's catalog with more detail. It does show a dome volume of -8.6 but also lists 10.5:1 for the 58 cc head. The 11.22:1 CR is based on a 50 cc head.
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PHAT69AMX
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Posted: Sep/19/2018 at 9:52pm |
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Would imagine since they say "Dome", and the value is Negative, Wiseco means Dish Volume...
I'd say it's generally a "given" piston companies assume: .003 shorter than stock nominal re-conditioned Connecting Rods .015 milled off of the stock nominal block deck height .015 milled off of the stock nominal cylinder head
Edited by PHAT69AMX - Sep/19/2018 at 9:56pm |
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AMXrated
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Posted: Sep/19/2018 at 9:59pm |
You're probably right since that makes the numbers more reasonable and closer to their specs. I'm going to email their tech dept. and confirm.
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304-dude wrote: